1. Field of the Invention
One embodiment of the present invention relates to pulse signal output circuits and shift registers.
2. Description of the Related Art
Transistors which are used for most flat panel displays typified by liquid crystal display devices and light-emitting display devices include silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon in many cases.
Although transistors including amorphous silicon have low field effect mobility, they can be formed over larger glass substrates. In contrast, although transistors including polycrystalline silicon have high field effect mobility, they need a crystallization process such as laser annealing and cannot be always formed over larger glass substrates.
In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used for transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor. For example, Patent Documents 1 and 2 disclose a technique by which a transistor is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and is used as a switching element of an image display device.
Further, such transistors including oxide semiconductors can be used as switching elements included in pixel portions and driver circuits of display devices such as liquid crystal displays, electroluminescent displays, and electronic paper. For example, Patent Document 3 discloses a technique in which a driver circuit is formed using a shift register and the like including a pulse signal output circuit, and a transistor including the above oxide semiconductor is used as a transistor in the pulse signal output circuit.
Transistors including oxide semiconductors in channel regions can have higher field effect mobility than transistors including amorphous silicon. Further, oxide semiconductor films can be formed at a temperature of 300° C. or lower by sputtering or the like, and a manufacturing process of the transistors including oxide semiconductor films is simpler than that of the transistors including polycrystalline silicon.